DHCP on Server 2012 R2 does not understand Fedora 25



  • My lab network runs DHCP from a Windows 2012 R2 server that is a typical DC (AD, DNS, DHCP).

    I just installed Fedora 25 and had pre specified the MAC address in Hyper-V because I wanted this system to get a DHCP reservation and I know that if I migrate it later with the MAC assignment dynamic, it will get a new MAC.

    Problem was that the Server 2012 R2 DHCP Server was looking at this huge 36 character unique ID from the Fedora system.

    A little search told me that back in Fedora 21 they had updated the DHCP client to be more RFC compliant and this is what broke things, because the Windows Server DHCP Server is not quite so RFC compliant.

    It is easy enough to fix. As root you will need to make the file /etc/dhcp/dhclient.conf and tell it to use the hardware for hte ID.

    echo "send dhcp-client-identifier = hardware;" >> /etc/dhcp/dhclient.conf
    


  • So coming back to this (because I was setting some more stuff up), it seems that Windows DHCP Server does understand the RFC, that is why it actually sees and uses the entire machine ID sent by default.

    It is other devices like my EdgeRouters that I have to use the MAC address.

    The only problem with the machine ID scheme is that I have no idea what it is before the machine is created.

    Granted this is mostly my problem because I always use DHCP reservations instead of static IP addresses (evil fucking things, those).

    I continue to force the new systems to use the hardware ID, because I want to only track the MAC address hex info.



  • Any idea if this gets fixed in Windows Server 2016?



  • Microsoft, always behind the curve. . . .



  • @DustinB3403 said in DHCP on Server 2012 R2 does not understand Fedora 25:

    Microsoft, always behind the curve. . . .

    Not behind, just not following standards. Different things. More like Cisco, Cisco is often ahead of the standards, but then refuses to follow them. That's why Cisco isn't 100% compatible with Gigabit Ethernet.



  • @scottalanmiller that still falls behind the curve, just because some things are ahead of the standard, doesn't mean that the things that are standard shouldn't be supported.



  • @DustinB3403 not really Scott is right here.



  • @scottalanmiller said in DHCP on Server 2012 R2 does not understand Fedora 25:

    Any idea if this gets fixed in Windows Server 2016?

    No idea, and is it something to fix? Or do you mean the ability to set DHCP reservations by the RFC compliant field?



  • @JaredBusch said in DHCP on Server 2012 R2 does not understand Fedora 25:

    echo "send dhcp-client-identifier = hardware;" >> /etc/dhcp/dhclient.conf

    Just tried this last night. It 2016, it has not changed.



  • I've been running several Fedora VMs for awhile now. I noticed that I can still reserve addresses so maybe the unique id doesn't have to be a MAC address format.

    When I right-click and select "Add to reservation", it adds without a peep.

    When I manually create or make any changes to a reserve address and I click on OK or Apply.
    A DHCP window prompt appears telling me that "The Unique Identifier you have entered may not be correct. Do you want to use this Identifier anyway?" So I then click Yes.

    After restarting the NetworkManager, VMs end up showing the IP that I assigned to them. And the Lease Expiration still show Reservation (active).

    When I add a reservation in powershell using Add-DhcpServerv4Reservation it doesn't even complain about the Identifier. So that annoying window prompt must be expecting MAC addresses because that's what Windows clients uses as a Identifier.

    Add-DhcpServerv4Reservation
    https://docs.microsoft.com/en-us/powershell/module/dhcpserver/add-dhcpserverv4reservation?view=win10-ps

    Add-DhcpServerv4Reservation `
    -ComputerName "dc01.ad.domain.local" `
    -ScopeId 192.168.1.0 `
    -IPAddress 192.168.1.250 `
    -ClientId "5d0d521d00042f1bc746647948d4b17538dde989207d" `
    -Name "fedora.ad.domain.local" `
    -Description "Reservation for Fedora ClientId"
    


  • @black3dynamite good to know. I had not tried this actually. Just hit google and found the above mentioned setting.



  • I've noticed this too.

    How I've been doing it, is letting DHCP give an address... then I look it up on the DHCP server.

    It ends up being something like "56cb31d80004e40ee72bdcaa42a59cbddc4fd763163e", where the first part "56cb31d8" is the last 4 of 8 pairs of the actual MAC address, and the rest of it I haven't figured out where that comes from yet.

    After I find it on the DHCP server, I can reserve the entire thing as I would a normal MAC address... when you get that Unique Identifier popup that @black3dynamite mentions above, you just hit yes or OK and all is well. It's just a warning, and it still reserves it properly after that.

    I haven't figured out how to get Fedora to register a regular MAC on a Windows DHCP server yet. I don't know which one is at fault for the weirdness.



  • @tim_g said in DHCP on Server 2012 R2 does not understand Fedora 25:

    I haven't figured out how to get Fedora to register a regular MAC on a Windows DHCP server yet. I don't know which one is at fault for the weirdness.

    1st post.... that was the entire point of the thread.



  • @tim_g said in DHCP on Server 2012 R2 does not understand Fedora 25:

    I've noticed this too.

    How I've been doing it, is letting DHCP give an address... then I look it up on the DHCP server.

    It ends up being something like "56cb31d80004e40ee72bdcaa42a59cbddc4fd763163e", where the first part "56cb31d8" is the last 4 of 8 pairs of the actual MAC address, and the rest of it I haven't figured out where that comes from yet.

    After I find it on the DHCP server, I can reserve the entire thing as I would a normal MAC address... when you get that Unique Identifier popup that @black3dynamite mentions above, you just hit yes or OK and all is well. It's just a warning, and it still reserves it properly after that.

    I haven't figured out how to get Fedora to register a regular MAC on a Windows DHCP server yet. I don't know which one is at fault for the weirdness.

    I've also seen some of my HP printers using an Identifier that starts with the MAC address and ends with zeros.



  • @tim_g said in DHCP on Server 2012 R2 does not understand Fedora 25:

    It ends up being something like "56cb31d80004e40ee72bdcaa42a59cbddc4fd763163e", where the first part "56cb31d8" is the last 4 of 8 pairs of the actual MAC address, and the rest of it I haven't figured out where that comes from yet.

    I found it. The last part of the Identifier is located in /etc/machine-id.



  • @jaredbusch said in DHCP on Server 2012 R2 does not understand Fedora 25:

    @tim_g said in DHCP on Server 2012 R2 does not understand Fedora 25:

    I haven't figured out how to get Fedora to register a regular MAC on a Windows DHCP server yet. I don't know which one is at fault for the weirdness.

    1st post.... that was the entire point of the thread.

    I see now. I don't know how I missed that!

    Anyways, thanks.





  • So coming back to this (because I was setting some more stuff up), it seems that Windows DHCP Server does understand the RFC, that is why it actually sees and uses the entire machine ID sent by default.

    It is other devices like my EdgeRouters that I have to use the MAC address.

    The only problem with the machine ID scheme is that I have no idea what it is before the machine is created.

    Granted this is mostly my problem because I always use DHCP reservations instead of static IP addresses (evil fucking things, those).

    I continue to force the new systems to use the hardware ID, because I want to only track the MAC address hex info.